Responses of diatoms and aquatic macroinvertebrates to deposited fine sediment in mesocosm channels as a basis for a new water quality guideline.

Water quality guidelines are an important tool for managing environmental pressures on freshwater streams. Currently, there is a lack of guideline values for fine sediment deposition in South Australian and, more broadly, Australian and overseas streams despite the potentially profound impacts of sediment deposition on aquatic communities. We used an outdoor recirculating stream mesocosm to assess the responses of freshwater diatoms and macroinvertebrates to fine sediment burial after six weeks following a pulsed event. Communities were exposed to five treatments of fine sediment deposition (inert sand) with an average depth between 1 and 20 mm, added on top of, and compared to, representative background sediments from a high-quality stream dominated by hard-bottom sediments and detritus. The most pronounced change was between the control and treatment one (1 mm sediment depth). Sediment deposition negatively affected diatom composition, with a decrease in functional diversity but a concurrent increase in taxonomic diversity arising from increased abundances of motile species. For macroinvertebrates, there was a clear negative response in the richness and abundance of the sensitive Ephemeroptera, Plecoptera and Trichoptera insect orders to fine sediment burial, and an associated higher macroinvertebrate drift propensity. This study highlights the importance of incorporating functional and behavioural responses in addition to taxonomic metrics when assessing biotic responses to stressors. This study recommends a new guideline which restricts additional fine sediment deposition to <1 mm in temperate South Australian freshwater streams, particularly those dominated by hard-bottom substrates and, or containing sensitive aquatic taxa. This threshold could act as an interim guideline for these stream types elsewhere in the absence of comparable studies.